The invention lies in the field of processes for the recovery of aluminum from its ores using crystallization procedures.
The present invention is an improvement in the process disclosed in the above-referenced patent applications, this latter process including the following process steps as illustrated in the flow diagrams of the referenced patent applications.
1. ROASTING PARTICULATE ALUNITE CONCENTRATE TO REMOVE WATER OF HYDRATION;
2. REMOVING SULFUR COMPOUNDS AND ALKALI METAL COMPOUNDS, INCLUDING POTASSIUM SULFATE, FROM THE ROASTED ORE EITHER BY THE PROCEDURE OF U.S. Ser. No. 453,234 in which the roasted ore is subjected to a reducing roast and then to an oxidizing roast followed by leaching with a solvent, such as, water or an alkali metal hydroxide, or by the process of U.S. Ser. No. 453,225 in which the roasted ore is leached directly in a solvent, such as, ammonium or alkali metal hydroxide;
3. RECOVERING SOME OF THE ALKALI METAL SULFATES AND/OR AMMONIUM SULFATE IN THE LEACH SOLUTION OF (2) BY CRYSTALLIZATION AS DISCLOSED IN U.S. Ser. No. 453,225 as potassium sulfate liquor which builds up with contaminants in the mother liquor from the crystallization is bled off;
4. digesting the residue from the leaching step with caustic solution to convert aluminum values in the residue to soluble aluminates;
5. precipitating silica from the resulting solution of step (4) to leave a solution of aluminates and a residue known as a desilication product consisting essentially of sodium aluminum silicate along with other impurities;
6. precipitating aluminum values from the solution of step (5);
7. circulating the mother liquor from step (6) to the digestion step while bleeding off contaminated sodium hydroxide; and
8. recovering product aluminum from the aluminum values of step (6).
Both alternatives of step (2) above are included in the expression used herein, "roasting and leaching the ore to remove water and compounds of sulfur and alkali metals.
This invention is an improvement directed to the conversion of waste products from the basic process of the referenced patent applications into usable reagents for the process so that the overall economy of the basic process is enhanced.
The sales price of a commercial grade metal recovered from an ore is dependent upon economic factors related to the process for recovering the metal from its ores. Such factors include amounts of reagents used, processing time, recovery and reuse of by-products rather than discarding them to waste, and other factors. The present invention enhances the economics of the basic process as it results in the recovery of an essential reagent, potassium hydroxide, from two materials ordinarily discarded as waste from the referenced process, i.e., potassium sulfate produced as a by-product in the first leaching step and impurity loaded sodium hydroxide from the digestion step.
The invention also makes possible the use of another conventionally discarded product, i.e., the desilication product from the desilication step, which may be used in this invention as an ion exchange agent for forming potassium hydroxide from waste potassium sulfate and sodium hydroxide.